Differential phase contrast x-ray imaging system and components

ABSTRACT

A differential phase contrast X-ray imaging system includes an X-ray illumination system, a beam splitter arranged in an optical path of the X-ray illumination system, and a detection system arranged in an optical path to detect X-rays after passing through the beam splitter.

CROSS-REFERENCE OF RELATED APPLICATION

This application claims priority to U.S. Provisional Application No.61/513,175, filed Jul. 29, 2011, and 61/620,140, filed Apr. 4, 2012, theentire contents of which are hereby incorporated by reference.

This invention was made with Government support of Grant No.DE-FG02-99ER54523, awarded by the Department of Energy; and Grant No.1R21EB012777-01A1, awarded by the Department of Health and HumanServices, The National Institutes of Health (NIH). The U.S. Governmenthas certain rights in this invention.

BACKGROUND

1. Field of Invention

The field of the currently claimed embodiments of this invention relatesto X-ray systems, and more particularly to differential phase contrastX-ray imaging systems and X-ray illumination systems.

2. Discussion of Related Art

X-ray differential phase-contrast (DPC) imaging relies on the refractionof the X-rays passing through an object. Since for hard X-rays therefraction angles are in the μ-radian range, the basic technique usedfor DPC imaging is to angularly filter with μ-radian resolution thetransmitted X-ray beam, thus converting the angular beam deviations fromrefraction into intensity changes on a conventional detector. Theangular filtering is done using X-ray optics such as crystals orgratings (see [1] for a recent review).

A fundamental advantage of DPC imaging is that it is sensitive todensity gradients in the measured object rather than to its bulk X-rayabsorption. In medical imaging for instance refraction has a contrastenhancing effect at tissue boundaries, which enables the detection ofsoft tissues which are otherwise invisible in conventional X-rayimaging. The ultra-small angle scattering occurring in micro-structuredsoft tissue such as cartilage, tendon, ligament or muscle has also avolume contrast enhancing effect [1-5]. Another benefit of DPC formedical imaging is that it can improve contrast and resolution atsimilar or lower dose than in conventional X-ray imaging. This ispossible because DPC uses X-rays that are not absorbed by the body andbecause the soft tissue refraction coefficients decrease with X-rayenergy much slower than the absorption ones. In particular, by using forDPC a spectrum with mean energy in the 50-80 keV range approximately,the soft tissue dose is minimized while refraction strongly dominatesover absorption [1, 6].

X-ray phase-contrast is also of interest for imaging and non-destructivecharacterization in material sciences, in particular as concerns low-Zmaterials. The structure and defects of materials ranging from polymers,to fiber composites, to wood, and to engineered bio-materials can beprobed on the micrometer scale using X-ray phase-contrast [7-9]. Some ofthe techniques used for X-ray phase-contrast can also be applied withneutrons [10]. Recently X-ray phase-contrast has gained attention infusion energy research, where the capability of refraction based imagingto measure the density gradients in an object can be used for thediagnostic of high density plasmas in inertial confinement fusion (ICF)and other high energy density physics (HEDP) experiments [11].

Until recently, research on X-ray DPC imaging has been done mostly atsynchrotrons, using crystal optics; the high intensity of thesynchrotron compensates for the low efficiency (less than a hundredth ofa %) of the crystal optics [1, 12]. Although there are efforts todevelop table-top synchrotrons [13], or to use narrow K_(α) lines fromconventional tubes [14], the crystal method has not yet entered thedomain of practical applications. It is thus of interest to develop moreefficient DPC methods and optics, that can work with conventionalmedical or industrial X-ray tubes.

A DPC method that can work with conventional X-ray sources is theTalbot-Lau shearing interferometry, in which micro-periodic optics suchas gratings are used to angularly filter the refracted X-rays withμ-radian resolution [15-17]. The Talbot interferometer includes first a‘beam-splitter’ (typically a π-shift phase grating), which divides (or‘shears’) through the Talbot effect the incoming beam into few μ-radianwide beamlets. The Talbot effect consists in a ‘replication’ of thegrating pattern by the wave intensity, at periodic distances along thebeam, called Talbot distances, d_(T)=k/η²·g²/(2λ) with λ the X-raywavelength, g the grating period, k=1, 2, . . . the order of thepattern, and η=1 for a π/2 phase shifting grating or for an absorptiongrating, and η=2 for a π phase grating [18]. The beam-splitter thuscreates at the ‘Talbot distance’ a micro-periodic fringe pattern, whichchanges shape (shifts) with respect to the unperturbed pattern when arefractive object is introduced in the beam. The differentialphase-contrast imaging consists thus in measuring the changes in thefringe pattern induced by the object, with respect to the patternwithout the object. To achieve μ-radian angular sensitivity at hardX-ray wavelengths, the period g must be in the μm range, resulting in aTalbot distance of a few tens of cm.

The fringe pattern can in principle be directly measured using amicroscopic pixel detector [17]. This is however quite inefficient. Formost practical applications, the fringe pattern changes are convertedinto intensity changes on a macroscopic pixel detector by introducing an‘analyzer’ absorption grating placed behind the beam-splitter and havingthe period of the Talbot pattern. Lastly, for such an interferometer tofunction with an extended spot X-ray tube, a ‘source’ absorption gratingis placed in front of the source, thus dividing it into an array ofquasi-coherent line sources [16-18].

The gratings are made by micro-lithography in thin Si wafers orphotoresist [19, 20]. The absorption gratings are difficult tofabricate; they are typically made by filling with gold the gaps inregular transmission gratings. The ‘grating shearing method’ describedabove has demonstrated performance similar to the crystal method atenergies below a few tens of keV [21].

This method is however less useful at energies above a few tens of keV.The reason is that it is difficult to fabricate micron-period absorptiongratings with the thickness required to block higher energy X-rays. Thisis illustrated in FIG. 1 with a plot of the Au thickness needed for 95%absorption, as a function of the photon energy. As seen, several hundredμm depth gratings would be needed in the range of interest for clinicalDPC imaging. Depending on the grating period, the present technologicallimit is however around 50-100 μm [19, 20, 22]. This limits the contrastof the grating shearing method for high energy X-rays, as illustrated inFIG. 1 by the fringe contrast computed for an interferometer having 30μm thick, 4 μm period Au analyzer grating (throughout this specificationwe used for X-ray phase-contrast and optics calculations the XWFP wavepropagation code [23] and the XOP optics package [24]).

A new type of optics is therefore needed to enable efficient DPC imagingat X-ray energies above a few tens of keV.

BACKGROUND REFERENCES

-   1. Shu-Ang Zhou and Anders Brahme, “Development of phase-contrast    X-ray imaging techniques and potential medical applications”,    Physica Medica 24, 129 (2008).-   2. Carol Muehleman, Jun Li, Zhong Zhong, Jovan G. Brankov and    Miles N. Wernick, “Multiple-image radiography for human soft    tissue”, J. Anat. 208, 115 (2006)-   3. Tetsuya Yuasa, Eiko Hashimoto, Anton Maksimenko, Hiroshi    Sugiyama, Yoshinori Arai, Daisuke Shimao, Shu Ichihara, Masami Ando,    “Highly sensitive detection of the soft tissues based on refraction    contrast by in-plane diffraction-enhanced imaging CT”, Nuclear    Instruments and Methods in Physics Research A 591, 546 (2008)-   4. J. Li, Z. Zhong, D. Connor, J. Mollenhauer and C. Muehleman,    “Phase-sensitive X-ray imaging of synovial joints”, Osteoarthritis    and Cartilage 17, 1193 (2009)-   5. Paola Coan, Juergen Mollenhauer, Andreas Wagner, Carol Muehleman,    Alberto Bravin, “Analyzer-based imaging technique in tomography of    cartilage and metal implants: A study at the ESRF”, European Journal    of Radiology 68, S41 (2008)-   6. R A Lewis, “Medical phase contrast X-ray imaging: current status    and future prospects”, Phys. Med. Biol. 49, 3573 (2004)-   7. F. Pfeiffer, M. Bech, O. Bunk, P. Kraft, E. F. Eikenberry, Ch.    Bronnimann, C. Grunzweig and C. David, “Hard-X-ray dark-field    imaging using a grating interferometer”, Nature Materials 7, 134    (2008)-   8. Yogesh S. Kashyap, P. S. Yadav, Tushar Roy, P. S. Sarkar, M.    Shukla, Amar Sinha, “Laboratory-based X-ray phase-contrast imaging    technique for material and medical science applications”, Applied    Radiation and Isotopes 66, 1083 (2008)-   9. Sheridan Mayo, Robert Evans, Fiona Chen and Ryan Lagerstrom,    “X-ray phase-contrast micro-tomography and image analysis of wood    microstructure”, Journal of Physics: Conference Series 186, 012105    (2009)-   10. M. Strobl, C. Grünzweig, A. Hilger, I. Manke, N. Kardjilov, C.    David, and F. Pfeiffer, “Neutron Dark-Field Tomography”, Phys. Rev.    Lett. 101, 123902 (2008)-   11. Jeffrey A. Koch, Otto L. Landen, Bernard J. Kozioziemski,    Nobuhiko Izumi, Eduard L. Dewald, Jay D. Salmonson, and Bruce A.    Hammel, “Refraction-enhanced X-ray radiography for inertial    confinement fusion and laser-produced plasma applications”, J. Appl.    Phys. 105, 113112 (2009)-   12. Heikki Suhonen, Manuel Fernandez, Alberto Bravin, Jani    Keyrilainen and Pekka Suorttia, “Refraction and scattering of X-rays    in analyzer based imaging”, J. Synchrotron Rad. 14, 512 (2007)-   13. Martin Bech, Oliver Bunk, Christian David, Ronald Ruth, Jeff    Rifkin, Rod Loewen, Robert Feidenhans and Franz Pfeiffer, “Hard    X-ray phase-contrast imaging with the Compact Light Source based on    inverse Compton X-rays”, J. Synchrotron Rad. 16, 43 (2009)-   14. Muehleman C, Li J, Connor D, Parham C, Pisano E, Zhong Z.,    “Diffraction-enhanced imaging of musculoskeletal tissues using a    conventional X-ray tube”, Acad. Radiol. 16, 918 (2009)-   15. J. F. Clauser, “Ultrahigh resolution interferometric X-ray    imaging,” U.S. Pat. No. 5,812,629 (1998)-   16. Pfeiffer, F., Weitkamp, T., Bunk, O., David, C., “Phase    retrieval and differential phase-contrast imaging with    low-brilliance X-ray sources”, Nature Physics 2, 258 (2006)-   17. Atsushi Momose, Wataru Yashiro, Yoshihiro Takeda, Yoshio Suzuki    and Tadashi Hattori, “Phase Tomography by X-ray Talbot    Interferometry for Biological Imaging”,-   Japanese Journal of Applied Physics 45, 5254 (2006)-   18. Timm Weitkamp, Christian David, Christian Kottler, Oliver Bunk,    and Franz Pfeiffer, “Tomography with grating interferometers at    low-brilliance sources”, Proc. SPIE 6318, 6318 (2006)-   19. C. David, J. Bruder, T. Rohbeck, C. Grunzweig, C. Kottler, A.    Diaz, O. Bunk, F. Pfeiffer, “Fabrication of diffraction gratings for    hard X-ray phase contrast imaging” Microelectronic Engineering 84,    1172 (2007)-   20. Elena Reznikova, Juergen Mohr, Martin Boerner, Vladimir Nazmov,    Peter-Juergen Jakobs, “Soft X-ray lithography of high aspect ratio    SU8 submicron structures”, Microsyst. Technol. 14, 1683 (2008)-   21. Martin Bech, Torben H Jensen, Robert Feidenhans, Oliver Bunk,    Christian David and Franz Pfeiffer, “Soft-tissue phase-contrast    tomography with an X-ray tube source”, Phys. Med. Biol. 54 2747    (2009)-   22. Tilman Donath, Franz Pfeiffer, Oliver Bunk, Waldemar Groot, et    al., “Phase-contrast imaging and tomography at 60 keV using a    conventional X-ray tube source”, Rev. Sci. Instrum. 80, 053701    (2009)-   23. Timm Weitkamp, “XWFP: An X-ray wavefront propagation software    package for the IDL computer language”, Proc. SPIE 5536, 181-189    (2004)-   24. M. Sanchez del Rio and R. J. Dejus, “XOP: recent developments,    in Crystal and Multilayer Optics”, Proc. SPIE 3448, 340 (1998)

SUMMARY

A differential phase contrast X-ray imaging system according to anembodiment of the current invention includes an X-ray illuminationsystem, a beam splitter arranged in an optical path of the X-rayillumination system, and a detection system arranged in an optical pathto detect X-rays after passing through the beam splitter. The detectionsystem includes an X-ray detection component. The beam splitter includesa splitter grating arranged to intercept an incident X-ray beam andprovide an interference pattern of X-rays. The detection system includesan analyzer grating arranged to intercept and block at least portions ofthe interference pattern of X-rays prior to reaching the X-ray detectioncomponent. The analyzer grating has a longitudinal dimension, a lateraldimension that is orthogonal to the longitudinal dimension and atransverse dimension that is orthogonal to the longitudinal and lateraldimensions. The analyzer grating includes a pattern of optically denseregions each having a longest dimension along the longitudinal dimensionthat are spaced substantially parallel to each other in the lateraldimension such that there are optically rare regions between adjacentoptically dense regions. Each optically dense region has a depth in thetransverse dimension that is smaller than a length in the longitudinaldimension. The analyzer grating is arranged with the longitudinaldimension at a shallow angle relative to incident X-rays and the shallowangle is less than 30 degrees.

An X-ray illumination system according to an embodiment of the currentinvention includes a poly-energetic X-ray source and a band-pass filterarranged in an optical path of X-rays from the poly-energetic X-raysource. The band-pass filter allows X-rays within a band of energies topass more strongly than X-rays outside the band of energies.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objectives and advantages will become apparent from aconsideration of the description, drawings, and examples.

FIG. 1 shows gold thickness needed for 95% absorption, as a function ofX-ray energy. Also shown the fringe contrast for a gratinginterferometer having 30 μm thick, 4 μm period Au analyzer. At energiesof clinical interest the analyzer becomes transparent to X-rays,drastically reducing the interferometer contrast.

FIG. 2A is a schematic illustration of a differential phase contrastX-ray imaging system according to an embodiment of the currentinvention.

FIG. 2B is a schematic illustration of a conventional, normal incidenceTalbot-Lau interferometer.

FIG. 3A is a schematic illustration of an X-ray illumination system thathas a dual-mirror band-pass filter according to an embodiment of thecurrent invention.

FIG. 3B shows computed optical transmission of a dual-mirror filter(FIG. 3A) obtained combining two Pt mirrors at 3 mrad incidence angle,of which the first is deposited on a 3 μm thick Mylar membrane. Alsoshown the shape of the contrast curve of an m=5, <E>=26 keV Talbotinterferometer.

FIG. 4 is a plot of Au thickness needed for 95% absorption, as afunction of X-ray energy.

FIG. 5A shows computed contrast for 5 μm period, m=1 interferometer of60 keV mean energy, using 100 μm thick Au source and analyzer gratingsat normal incidence and at 10° incidence to contrast an embodiment ofthe current invention with a conventional system.

FIG. 5A is similar calculation as in FIG. 5A, but for interferometer of120 keV design energy, using 100 μm thick Au source and analyzergratings at 7° incidence. The grayed part of the curve represents lowenergy peaks that are removed by absorption of the low energy photons inthe object or using a separate spectral filter.

FIG. 6 is a schematic illustration of a differential phase contrastX-ray imaging system according to an embodiment of the current inventionthat has a large field of view.

FIG. 7A shows a Moiré pattern and intensity profile obtained withglancing angle) (22.5° Talbot-Lau interferometer and with spectrum of˜43 keV mean energy according to an embodiment of the current invention.

FIG. 7B shows similar data, but for normal incidence interferometer.

FIG. 7C shows Moiré fringe shifts produced by a 12 mm nylon rod withtilted grating interferometer according to an embodiment of the currentinvention. The right panel shows the X-ray spectrum for FIGS. 7A and 7B.

FIG. 8A is a schematic illustration of a differential phase contrastX-ray imaging system according to an embodiment of the current inventionthat has glancing angle gratings for phase-contrast imaging and alaterally graded multilayer mirror for quasi-monochromatic spectralfiltering.

FIG. 8B is a schematic illustration of a differential phase contrastX-ray imaging system according to an embodiment of the current inventionthat is similar to the embodiment of FIG. 8A, but uses a micro-periodicmirror instead of the source grating.

FIG. 9 shows a computed spectrum of 300 kVp W anode tube aftertransmission through 200 mm of soft tissue and 200 μm Cu. Also shown thespectrum after reflection on a Au mirror at 1.1 mrad, together with thecontrast of an m=5 interferometer having 100 μm thick Au gratings at 10°incidence angle.

FIG. 10 is a schematic illustration of a Talbot-Lau gratinginterferometer with conventional X-ray source.

FIG. 11A-11D show simulated phase-scan curve (a), refraction enhancedimage (b), phase-gradient image (c), and attenuation image (d), of 1 mmBe rod in water medium. We assumed an m=3, <E>=20 kV, 10 μm periodsymmetric interferometer of 2.4 m length and a W anode tube as source. A100 μm diameter Au wire was also included as a contrast reference. Atypical rocking crystal curve in the ABI method is also plotted in FIG.11A.

FIG. 12 shows computed refraction angles for IFE capsule model at 22keV. The attenuation image is also shown as inset.

FIG. 13 shows computed refraction angles for small joint phantom at 25keV. The layout of the joint phantom is shown at the top.

FIG. 14A shows dependence of angular width on interferometer length, for<E>=25 keV, m=3. Also shown the angular width for Z=2 m and m=7 (dottedline).

FIG. 14B shows grating period variation with M_(T) for <E>=25 keV, Z=2m, m=3.

FIG. 15 shows computed contrast as a function of energy and Talbot orderfor 2 m interferometer of <E>=25 keV. Also shown the shape of the powerspectrum of a W anode tube at 35 kV.

FIG. 16 shows computed Talbot pattern at the analyzer position for them=5, E=<25 keV> interferometer in FIG. 15, at energies of 19, 25 and 37keV. The position of the analyzer grating bars is shown by horizontallines. For reference the m=5 contrast curve in FIG. 15 is also replottedat the top.

FIG. 17A shows a normalized power spectrum of Rh tube filtered with 30μm Rh absorber; also shown the contrast of an m=7, <E>=20 keV symmetricinterferometer.

FIG. 17B shows the spectrum corresponding to FIG. 17A after low-passfiltering by reflection on a Pt mirror at 3.5 mrad.

FIGS. 18A-18D show images of small joint phantom using different sourcespectra: a) W anode tube at 35 kV, m=3; b) K-edge filtered Rh tubespectrum at 40 kV, m=7; c) Total reflection mirror filtered Rh tubespectrum, m=7; and d) Multilayer mirror filtered Rh tube spectrum, m=7.

FIG. 19 shows a Moiré image of IFE capsule with Ag—Kα backlighting. Theimage of a 50 μm diameter opaque sphere is also shown in the top rightcorner as a contrast reference.

FIG. 20 is a schematic illustration of a differential phase contrastX-ray imaging system according to an embodiment of the currentinvention.

FIG. 21A shows a computed refraction enhanced image of large jointphantom using separate, absorption source grating and mirror filtering.

FIG. 21B shows a phantom image obtained assuming a micro-periodic mirroras reflective source grating.

DETAILED DESCRIPTION

Some embodiments of the current invention are discussed in detail below.In describing embodiments, specific terminology is employed for the sakeof clarity. However, the invention is not intended to be limited to thespecific terminology so selected. A person skilled in the relevant artwill recognize that other equivalent components can be employed andother methods developed without departing from the broad concepts of thecurrent invention. All references cited anywhere in this specification,including the Background and Detailed Description sections, areincorporated by reference as if each had been individually incorporated.

Some embodiments of the current invention can use commercially availablemicro-periodic gratings tilted at glancing incidence (incidence angles αin the range from a few degrees to a few tens of degrees), to makeTalbot-Lau differential phase-contrast (DPC) interferometers up to veryhigh X-ray energy (100 keV and higher). Some embodiments of the currentinvention may also include grazing incidence mirrors in conjunction withthe tilted gratings that help to produce a quasi-monochromatic X-rayspectrum and/or to improve the coherence of the radiation incident onthe gratings.

Some applications, according to some embodiments of the currentinvention, can include medical X-ray imaging where refraction andultra-small-angle scatter (USAXS) have been shown to strongly enhancethe visibility of soft tissues, such cartilage, tendon, blood vesselwalls, brain tissue, micro calcifications, and tumors. Some embodimentsof the current invention can work with high energy X-rays and with highpower, extended spot medical X-ray tubes, thus enabling X-rayphase-contrast imaging of tissues deep in the human body. Examples ofpossible medical applications are ‘X-ray biopsy’ systems that may enableearly cancer detection for organs deep in the body, such as theprostate, lung, pancreas, or brain.

In addition, other applications of some embodiments of the currentinvention can be used in the field of engineered tissues, materialsciences and materials based on nanostructures, industrialnon-destructive testing (NDT), and security screening and energyresearch, for example. In NDT for instance, phase-contrast imaging withX-rays around 100 keV could enable improved detection of cracks andmicro-structural fatigue damage in critical components such as airplanewings and fuselage. However, the general concepts of the currentinvention are not limited to these particular examples.

The main imaging modalities for soft tissues are MRI, ultrasound, andX-rays. However, while MRI and ultrasound provide good soft tissuecontrast, their spatial resolution is limited. Conventional (attenuationbased) X-ray imaging on the other hand has good spatial resolution, butpoor soft tissue contrast.

In recent years a new X-ray imaging modality called differentialphase-contrast (DPC) and based on X-ray refraction and ultra-small anglescatter has been explored that offers both good soft tissue contrast andhigh spatial resolution. These capabilities arise from the sensitivityof DPC to small-scale density gradients in the object rather than to itsbulk absorption. This enhances the contrast for tissue boundaries andfor micro-structured tissues such as cartilage, tendon, ligament ormuscle. In addition, recent studies show that DPC can provide sensitivedetection of tumors in a variety of organs, from the breast, to theliver and to the lung. There is thus a rapidly growing spectrum ofpossible medical applications of X-ray DPC [1]. In addition, there couldbe many novel applications of X-ray phase-contrast in non-destructivetesting and material sciences.

DPC imaging works by using X-ray optics to angularly filter therefracted component in the transmitted radiation. Recently a veryefficient DPC method was developed that enables the use of conventionalX-ray tubes. The method is based on the Talbot-Lau interferometer setupin which micro-periodic absorption and transmission gratings are used toangularly filter the refracted X-rays [2,3].

Due to technological limits in the fabrication of thick micro-periodicgratings [4,5], the conventional Talbot-Lau interferometer usinggratings at normal incidence has insufficient fringe contrast orvisibility at X-ray energies above a few tens of keV [2-4]. X-rays abovea few tens of KeV are however needed to penetrate large body parts. Thesame limitation occurs in industrial or material research applicationsof DPC imaging.

Some embodiments of the current invention are directed to a new type ofX-ray imaging systems based on Talbot-Lau interferometers havingglancing incidence micro-periodic gratings, or combinations of glancingincidence gratings and mirrors. These systems can enable high resolutionDPC imaging with X-rays up to 100 keV or higher and using conventional,extended spot X-ray tubes. The systems described according to someembodiments of the current invention also have sufficiently large 2-Dfields of view (order of 2×7 cm for a single interferometer) to enablemost practical applications.

Some embodiments of the current invention can be used in combinationwith and/or further develop concepts described by the current inventorsin MICRO-PERIODIC MIRROR BASED SYSTEMS FOR PHASE-CONTRAST IMAGING WITHHARD X-RAYS [7]. This previously reported system can provide DPC imagingat high energy, but one distinction is that the field of view is limitedto a few hundred μm in one dimension.

FIG. 2A provides a schematic illustration of a differential phasecontrast X-ray imaging system 100 according to an embodiment of thecurrent invention. The differential phase contrast X-ray imaging system100 includes an X-ray illumination system 102, a beam splitter 104arranged in an optical path 106 of the X-ray illumination system 102,and a detection system 108 arranged in an optical path 110 to detectX-rays after passing through the beam splitter 104. The detection system108 includes an X-ray detection component 112. The beam splitter 104includes a splitter grating, as is shown in the embodiment of FIG. 2A,arranged to intercept an incident X-ray beam and provide an interferencepattern of X-rays.

The detection system 108 also includes an analyzer grating 114 arrangedto intercept and block at least portions of the interference pattern ofX-rays prior to reaching the X-ray detection component 112. The analyzergrating 114 has a longitudinal dimension, a lateral dimension that isorthogonal to the longitudinal dimension, and a transverse dimensionthat is orthogonal to the longitudinal and lateral dimensions. Theanalyzer grating 114 has a pattern of optically dense regions, eachhaving a longest dimension along the longitudinal dimension and spacedsubstantially parallel to each other in the lateral dimension such thatthere are optically rare regions between adjacent optically denseregions. Each optically dense region has a depth in the transversedimension that is smaller than a length in the longitudinal dimension.The analyzer grating 114 is arranged with the longitudinal dimension ata shallow angle α relative to incident X-rays such that the shallowangle α is less than 30 degrees. As is illustrated in the embodiment ofFIG. 2A, the longitudinal dimension of the analyzer grating 114 isoriented substantially along the optical path 110 (which can be theoptical axis, for example), except tilted at the shallow angle α. (Thiswill also be referred to as a glancing angle.)

In an embodiment of the current invention, each optically dense regionhas a depth in the transverse dimension that is smaller than a length inthe longitudinal dimension by at least a factor of two. In anembodiment, each optically dense region has a depth in the transversedimension that is smaller than a length in the longitudinal dimension byat least a factor of ten. In a further embodiment, each optically denseregion has a depth in the transverse dimension that is smaller than alength in the longitudinal dimension by at least a factor of onehundred.

In an embodiment of the current invention, the shallow angle α is lessthan 25 degrees and greater than 5 degrees. In another embodiment, theshallow angle α is less than 15 degrees and greater than 3 degrees. Anembodiment of the current invention is directed to medical applications.Since it is difficult to produce few-micron period gratings with morethan ˜100 μm Au absorber thickness, inclining the gratings at an anglein the 5-25° range makes for 200-1000 μm effective Au thickness. As isshown in FIG. 4, this thickness enables >90% X-ray absorption (and thushigh interferometer contrast) over the ˜40 keV-110 keV energy range, ofinterest for medical phase-contrast imaging deep in the body. Anotherembodiment is directed to industrial or non-destructive testing (NDT)applications. Using glancing angles in the 3-15° range, the effective Authickness is in the 400-2000 μm range, which makes for good X-rayabsorption and interferometer contrast in the ˜100 keV-250 keV energyrange of interest for industrial NDT applications.

In an embodiment of the current invention, the splitter grating 104 is areflection grating (not shown in FIG. 2A). A reflection grating such asdescribed in Ref. [7], which is incorporated herein by reference, can beused according to some embodiments of the current invention. In anembodiment of the current invention, the splitter grating 104 is atransmission grating, as is illustrated schematically in FIG. 2A.According to an embodiment of the current invention in which thesplitter grating 104 is a transmission grating, similar to analyzergrating 114, such an embodiment of the analyzer grating has alongitudinal dimension, a lateral dimension that is orthogonal to thelongitudinal dimension, and a transverse dimension that is orthogonal tothe longitudinal and lateral dimensions. The splitter grating 104 inthis embodiment has a pattern of optically dense regions, each having alongest dimension along the longitudinal dimension and being spacedsubstantially parallel to each other in the lateral dimension such thatthere are optically rare regions between adjacent optically denseregions. Each optically dense region has a depth in the transversedimension that is smaller than a length in the longitudinal dimension.The splitter grating 104 is arranged with the longitudinal dimension ata shallow angle α relative to incident X-rays such that it is less than30 degrees. In some embodiments, the splitter grating 104 can be similarin construction as the analyzer grating 114 and arranged similarly at ashallow angle α as described above with respect to the analyzer grating114, although placed at a different position along the optical axis.

FIG. 2B is a schematic illustration of a conventional differential phasecontrast X-ray imaging system that can be contrasted with thedifferential phase contrast X-ray imaging system 100 according to anembodiment of the current invention. In such a conventional system thatis based on a Talbot-Lau interferometer, the gratings are arrangedorthogonal to, and in some cases at slightly off-orthogonal angles tothe optical axis along which a beam of X-rays travels. As is illustratedin FIG. 2B, the longitudinal direction of the source, beam-splitter andanalyzer gratings are all in the vertical direction of the illustration.The thickness of the grating t is the maximum depth of correspondingoptically dense regions, such as parallel lines of gold or other high-Zmaterial separated by regions of low-Z material, such as a siliconsubstrate. According to the conventional approach, one would have toincrease the depth of the optically dense regions to operate with higherenergy X-rays in order to sufficiently block the higher energy X-rayswith the optically dense regions.

The current inventors recognized, and through experimentationdemonstrated, that such gratings could be oriented as is illustrated inFIG. 2A such that incident X-rays would have to travel through muchlonger paths in the optically dense layers than the thickness t of thegrating. Depending on the particular gratings, the paths the X-raysfollow through optically dense material in the gratings can be orders ofmagnitude greater than the thickness t. However, since the gratingscause diffraction and interference effects due to the wave nature of theX-rays, it was difficult to predict either theoretically and/ornumerically how such a change in geometry of the diffraction gratingswould affect the X-ray beam. The current inventors thus developed anddemonstrated the differential phase contrast X-ray imaging system 100,as illustrated schematically in FIG. 2A, by experimentation.

As used herein, the term “block” X-rays is intended to mean thatsufficient attenuation is achieved relative to X-rays that pass throughthe optically rare regions of the grating to permit a useful contrastfor the particular application. It is not intended to require absolutely100% attenuation.

The splitter grating 104 and the analyzer grating 114 are arranged witha separation determined according to Talbot-Lau conditions according tosome embodiments of the current invention. In some embodiments, thesplitter grating 104 and the analyzer grating 114 have grating patternsthat are determined according to Talbot-Lau conditions.

The X-ray illumination system 102, according to some embodiments of thecurrent invention can include an X-ray source 116, and a source grating118 arranged in an optical path between the X-ray source 116 and thebeam splitter 104. The source grating 118 provides a plurality ofsubstantially coherent X-ray beams when X-ray source 116 is a spatiallyextended source of X-rays, as is illustrated schematically in FIG. 2A.However, the broad concepts of the current invention are not limited tothe particular embodiment illustrated in FIG. 2A. The X-ray illuminationsystem 102 can include combinations of one or more gratings and mirrors,including both transmission and/or reflection gratings.

FIG. 3A is a schematic illustration of an X-ray illumination system 200according to an embodiment of the current invention. The X-rayillumination system 200 can be used as part of the differential phasecontrast X-ray imaging system 100 and/or any of the variations describedabove and/or can be used in conventional systems such as thatillustrated in FIG. 2B, for example. For example, the X-ray illuminationsystem 200 can be used for, or as a portion of, the X-ray illuminationsystem 102. However, the X-ray illumination system 200 is not limited toonly these particular applications.

The X-ray illumination system 200 has a poly-energetic X-ray source 202and a band-pass filter 204 arranged in an optical path of X-rays 206from the poly-energetic X-ray source 202. The band-pass filter 204allows X-rays within a band of energies to pass more strongly thanX-rays outside the band of energies. In an embodiment of the X-rayillumination system 200, the band-pass filter 204 includes a high-passX-ray mirror 208 that reflects a first portion 210 of an incident beamof X-rays 206 that have energies less than a lower pass-band energy andallows a second portion 212 of the incident beam of X-rays to passtherethrough. The band-pass filter 204 also includes first beam stop 214arranged to intercept and at least attenuate the first portion 210 ofthe incident beam of X-rays 206 that have energies less than the lowerpass-band energy, a low-pass X-ray mirror 216 that reflects a portion218 of the second portion 212 of the incident beam of X-rays 206 afterpassing through the high-pass X-ray mirror 208 that have energies lessthan a upper pass-band energy, and a second beam stop 220 arranged tointercept and at least attenuate X-rays that miss the high-pass X-raymirror 208 prior to reaching the second beam stop 220. The first andsecond beam stops (214, 220) are arranged to allow a beam of X-rays 222having energies between the upper pass-band energy and the lowerpass-band energy to pass therethrough. The band-pass filter 204 is notlimited to the particular example illustrated in FIG. 3A. In otherembodiments, more than three mirrors can be used, for example. The X-rayillumination system 200 provides a more monochromatic beam of X-raysthan that of the X-ray source 202. Furthermore, reflection and/ortransmission gratings can be used in combination with the band-passfilter 204 to improve coherence of the X-rays from the poly-energeticX-ray source 202. In further embodiments, a combination of high-passmirrors and at least one low-pass mirror can provide combined improvedcoherence and chromaticity of X-rays from the poly-energetic X-raysource 202.

The low-pass X-ray mirror can be a membrane X-ray mirror, for example,that has a reflecting layer that is a high-Z material on a support layerthat is a low-Z material. Z is the atomic number. The term “high-Zmaterial” is intended to mean materials that include atomic elementswith Z at least 42 (for example, but not limited to Rh, Pt, and/or Au)so as to have a relatively strong reflectivity for the X-rays. The term“low-Z material” is intended to mean materials that include atomicelements with Z less than 14 (for example, but not limited to C, Si,quartz, and/or glass) so as to have a relatively low reflectivity forthe X-rays.

The following are some new elements according to some embodiments of thecurrent invention, as contrasted to conventional system:

-   -   i) The use of micro-periodic gratings having the absorbing bars        tilted at a glancing angle along the direction of the incident        radiation as in FIG. 2A    -   The tilting of the gratings is a modification of the        conventional Talbot-Lau interferometer at normal incidence (FIG.        1B). Although this modification appears simple, it is difficult        to foresee theoretically that a glancing incidence Talbot-Lau        interferometer will work with extended sources. We arrived at        this idea following the concept of ‘physical period’ mirrors and        could verify that it works only through direct experimentation.    -   ii) The use of micro-periodic gratings at glancing angle in        conjunction with simple or micro-periodic X-ray mirrors.    -   As further discussed, one embodiment of the current invention        uses a simple total reflection X-ray mirror at grazing incidence        to select the spectral region where the interferometer has        highest contrast. In another embodiment the source grating is        replaced by a micro-periodic mirror in the ‘physical period’        geometry described in Ref. 7, which combines in a single optical        element the spectral filtering and the production of        quasi-coherent radiation.    -   iii) The use of spectral band-pass multilayer X-ray mirrors in        conjunction with tilted gratings.    -   In another embodiment of the invention, graded multilayer        mirrors are used as a spectral filter or as a ‘source grating’,        for further improved interferometer contrast and angular        sensitivity.    -   iv) The use of energy-resolving detectors to select the spectral        region of maximal interferometer contrast.

The phase-contrast imaging system of the example illustrated in FIG. 2Aincludes three micro-periodic gratings in a Talbot-Lau interferometerconfiguration, tilted at equal glancing angles α, in the range from afew degrees to a few tens of degrees. The period of the gratings can bea few μm (e.g., but not limited to, g0=g1=g2=5 μm) and the gratinginter-distances and periods follow the equations of the normal incidenceTalbot-Lau interferometer. The first grating is a ‘source grating’,which produces an array of quasi-coherent line sources from an extendedincoherent source. The second grating is a beam-splitter which producesa high contrast fringe pattern (the ‘Talbot pattern’) at the analyzerlocation when illuminated through the source grating. Lastly, ananalyzer grating is used to transform changes in the Talbot pattern intointensity changes on a 2-D X-ray detector.

The system works similarly to the conventional, normal incidenceTalbot-Lau interferometer [2,3], sketched for reference in FIG. 2B. Whena refractive object is placed in the X-ray beam (“Object” in FIG. 2A) itperturbs the Talbot pattern produced by the beam-splitter. The analyzertransforms this perturbation into an intensity change on the detector,which enables imaging and quantifying the X-ray refraction and scatterinduced by the object.

The source and analyzer gratings can be conventional, commerciallyavailable absorption gratings made, for example, by filling the gaps ina silicon or photoresist grating with gold, as described in Refs. [5,6]. The beam-splitter can be a π-shift phase grating, also can also bemade in the conventional manner.

However, according to some embodiments of the current invention, thegratings are tilted at a glancing angle and have the absorbing barsalong the direction of the incident radiation, as shown schematically inFIG. 2A. Our experiments demonstrated that this modification of theTalbot-Lau setup solves in a simple and practical manner the problem ofDPC imaging at high energy.

Indeed, an obstacle to the use of normal incidence Talbot-Lauinterferometers at high energy is the practical limit in the thicknessof small period source and analyzer gratings [5,6]. To obtain highinterferometer contrast or visibility the absorbing bars of the sourceand the analyzer gratings must be strongly attenuating (typically around90-95%). At the same time, the X-ray absorption of any materialdecreases rapidly as the X-ray energy is increased. This is illustratedin FIG. 4 which shows, as a function of energy, the Au thickness neededto absorb 95% of the incident X-rays. As one can see, the thicknessneeded for efficient absorption at E>40 keV is >several hundred μm.

At present, however, it is not technologically possible to makeabsorption gratings with a few micron periods and several hundred μmthickness. The current limit in the grating aspect ratio (ratio betweenbar thickness and width) is around 50, while, as shown above,aspect-ratios of several hundred would be needed to make high contrastinterferometers for high energy. This fact is confirmed by experiment.Thus, attempts to build a Talbot-Lau interferometer of 60 keV meanenergy using normal incidence gratings had little success: the fringecontrast was of only several %. The same effect can be seen in FIGS. 5Aand 5B below. Note however that phase gratings for high energy caneasily be made, since they need to be much thinner [2,3,7,8].

Some embodiments of the current invention can provide a simple,practical and also economical solution to this problem: by tilting thegratings at a glancing angle α, the effective absorber thickness in theX-ray path increases to t/sin(α), with t the physical or normalincidence thickness of the grating. For instance at α˜10° the effectivethickness increases by a factor of 6. Thus, a 100 μm thick, 5 μm periodgrating, which is within the present technological capability, appearsas a grating of 600 μm thickness when tilted at a glancing angle of 10°in the direction of the radiation.

The physical thickness of the beam-splitter is simply that required toproduce a π-phase shift at the desired design energy E₀, when viewed byX-rays incident at an angle α; for instance, if t(0) is the thicknessneeded for normal incidence operation at E₀, the thickness required atglancing incidence α, is t*sin(α).

Some embodiments of the current invention can enable, in this way,building high contrast Talbot-Lau interferometers up to very high X-rayenergy. This is shown in FIG. 5A which plots the computed contrast as afunction of energy for an interferometer having 100 μm thick gratings atnormal incidence, and at 10° glancing incidence angle. The beam-splitteris a Ni phase grating having t(0)=20 μm for a mean or ‘design’ energy of60 keV. The duty-cycle (gap width/period) of the source grating is 37%and the Talbot order is m=1.

As shown in FIG. 5A, tilting the gratings produces a dramatic contrastincrease for energies above 40 keV approximately. In particular, goodcontrast obtains in the 40-70 keV range, which is of high interest formedical phase-contrast imaging because in this range the soft tissuedose is at a minimum [1]. In addition, appreciable contrast obtains alsoabove the Au K-edge at 80 keV.

As one can see for example with reference to FIG. 5A, some embodimentsof the current invention can provide high contrast interferometers foreven higher X-ray energies. This is illustrated in FIG. 5B which plotsthe computed contrast for an m=1 interferometer having 100 μm thick Ausource and analyzer gratings, tilted at 7°. The phase grating in thiscase is made of gold and has t(0)=10 μm, for a 120 keV design energy.The source grating duty-cycle is 37%. As seen, a broad band of highinterferometer contrast obtains in the region ˜90-130 keV. Thecapability for operation at these high energies makes some embodimentsof the current invention also of strong interest for NDT and securityapplications.

At the same time, some embodiments of the current invention can allowone to obtain interferometers with sufficiently large fields of viewsfor medical and other practical applications. For instance, acommercially available 70×70 mm analyzer grating would enable one toobtain a 12×70 mm field of view at 10° incidence and a 9×70 mm field ofview at 7° incidence. In addition, it is easy to make high energyimaging systems with larger fields of view by stacking multiple tiltedgratings, as is illustrated schematically in FIG. 6.

As mentioned, although the modification of the Talbot-Lau interferometeraccording to some embodiments of the current invention appears at afirst look straightforward, it is nevertheless difficult to predicttheoretically or computationally that a glancing incidence setup withthe grating bars oriented along the direction of the incident X-rays asin FIG. 2A, can work with a spatially extended X-ray source. Whileglancing angle grating Talbot interferometers have been discussed in theliterature [10,11], the grating bars have been always orientedperpendicularly to the direction of the incoming radiation (i.e., the‘effective period’ geometry discussed in Ref. 7). In this geometry,however, the grating contrast at high energy does not improve whentilting the gratings, because the effective X-ray path through theabsorber decreases instead of increasing.

We thus developed embodiments of the current invention experimentallyusing a Talbot-Lau interferometer having gratings tilted at a glancingangle of 22.5° and operated at ˜43 keV mean energy. All the gratings hadequal period of 10 μm, with the source grating having 55 μm thick Aubars and the analyzer 100 μm thick Au bars. The phase grating was a 23μm thick Si grating tilted at the same angle of 22.5°. All the gratingshad 50% duty cycle. The interferometer was operated in the first Talbotorder using as X-ray source an extended spot W anode tube at 60 kVp. Toobtain a spectrum with around 43 keV mean energy the tube output wasfiltered with a 100 mm thick water layer and with a 65 μm Cu. Thecomputed spectrum incident on the gratings is shown in the right panelof FIG. 7C.

A Moiré fringe pattern produced by the tilted gratings is shown in theleft panel of FIG. 7A, while a lineout through the pattern is shown inthe right panel. The fringe contrast is defined as:V=(I_(max)−I_(min))/(I_(max)+I_(min)). As one can see, using tiltedgratings can provide good interferometer contrast (V-25%) at high X-rayenergy. Even higher contrast would be obtained with a 100 μm thicksource grating, similar to the analyzer one.

For comparison, FIG. 7B illustrates the limited contrast that can beobtained with Talbot-Lau interferometers using normal incidencegratings. The Moiré pattern in this case has been obtained using 5.4 μmperiod gratings, with source and analyzer gratings having nominally 100μm thickness, which is about the technological limit for this period.The phase grating was a 15 μm thick Ni grating designed for 40 keV meanenergy. The incident spectrum was the same as in FIG. 7A. As can beseen, the best achievable normal incidence contrast is more than twicelower (V˜11%) than at glancing incidence. In addition, the contrast ofthe glancing incidence interferometer can easily be pushed to evenhigher values by further tilting the gratings.

Lastly, FIG. 7C demonstrates that the glancing angle Talbot-Lauinterferometer performs phase-contrast measurements similar to thenormal incidence one. The left panel in FIG. 7C shows the perturbedMoiré pattern obtained with the tilted gratings when imaging a nylon rodof 12 mm diameter. (The opaque object in the image is a Sn wire of 1.5mm diameter). As can be seen in FIG. 7C, while the nylon rod is almosttransparent to X-rays, it nevertheless produces strong Moiré fringeshifts near its edges.

In conclusion, our experimental results indicate that imaging systemsbased on glancing incidence Talbot-Lau interferometers offer a simplebut powerful solution to differential phase-contrast imaging at highX-ray energy. In addition, since the above results were obtained with athick water layer in the X-ray path, they directly demonstrate that thesystems in the Invention can work for phase-contrast imaging of thickbody parts using conventional X-ray tubes. So far, this possibility wasdemonstrated only using synchrotron X-ray sources.

The tilted grating Talbot-Lau interferometer concept described hereincan be directly applied for X-ray phase-contrast imaging at high energywithout any further development. This is particularly the case forapplications in which the angular sensitivity of m=1 Talbot-Lauinterferometers is sufficient (the angular sensitivity increases withthe Talbot order m as √m, with m=1, 3, 5 . . . ). Example of suchsituations would be ultra-small angle scattering (USAXS) imaging systemsfor non-destructive testing and studies of micro/nano structured matterin material sciences, nanotechnology, or industry. High energy m=1tilted grating systems could also be of interest for medical bonephase-contrast imaging, since bone is a strong USAXS scatterer.

For refraction based soft tissue imaging at high energy the angularsensitivity of m=1 interferometers is likely too low because therefraction angles scale as 1/E². To make high energy Talbot-Lauinterferometers that also have high angular sensitivity, one must workin higher (m>3) Talbot orders. At high-m however the spectral region ofgood contrast gets narrower (width˜1/m) and spectral filtering can beemployed to maintain good interferometer contrast [8]. Thus combiningthe glancing angle grating concept with the X-ray mirror filteringconcept can be useful for some applications.

Another alternative embodiment would be to use energy resolvingdetectors to select the spectral region of high interferometer contrast.In FIG. 5B, this would be for instance the region between 90 keV and 130keV approximately. 2-D pixilated detectors such as CdTe arrays existnowadays that have high energy resolution, high quantum efficiency andgood photon counting capability, at energies up to a few hundred keV.This novel approach is of particular interest for situations that cantolerate a higher radiation dose, such as in industrial applications,since a large flux of photons outside the region of high interferometercontrast would not be detrimental.

Other alternative embodiments can include the following two basicvariations:

-   -   1) High energy phase-contrast imaging systems using only        glancing angle gratings, such as in FIG. 2A.    -   One embodiment for this variation is a high energy m=1 DPC        imaging system using an energy resolving detector to        discriminate the photons outside the region of high contrast. An        example application for such a system would be phase-contrast        based non-destructive testing of composite metallic parts in the        aerospace and aviation industry.    -   2) High energy phase-contrast imaging systems combining glancing        incidence gratings with total reflection or Bragg reflection        (multilayer) mirrors, such as in FIGS. 8A and 8B.    -   The mirror can be a simple, non-patterned mirror that serves        only as spectral filter (FIG. 8A), or it can be a        micro-periodically patterned mirror having strips parallel to        the incident X-rays (the ‘physical period’ geometry described in        Ref. 7) that would replace the source grating (FIG. 8B). In the        latter case the mirror would serve simultaneously as spectral        filter and spatial filter, thus reducing the number of optical        elements and simplifying the setup. Further, the mirror can be        either a total reflection mirror working at angles around 1-1.5        mrad, or a graded multilayer mirror working at larger angles of        several mrad.

An embodiment of such a system would be an m=5 interferometer for thetungsten K-shell line emission between ˜60-70 keV. Thisquasi-monochromatic emission can be made very bright using W anode tubesat high voltage (few hundred kV). In addition, as mentioned, this energyregion is ideal for medical phase-contrast imaging deep in the humanbody.

The principle of this embodiment is sketched in FIG. 9. The totalreflection on the mirror effectively cuts off the high energy portion ofthe spectrum, which would contribute to the dose without contributing tothe phase contrast image [8]. The low energy part of the spectrum is cutoff by an absorption filter. The mirror/filter combination produces thusa quasi-monochromatic band of radiation that matches well the contrastcurve of an m=5 Talbot-Lau interferometer (FIG. 9).

The filtering mirror can also be a laterally graded synthetic multilayermirror, which can reflect only a narrow band between ˜60-70 keV,allowing thus to work in even higher Talbot orders (e.g. m=9) and thusto achieve even higher angular sensitivity and interferometer contrast.Lastly, the mirror can be micro-periodically patterned and thus fulfillsimultaneously the function of spectral filter and of source grating.

The field of view of systems combining glancing angle gratings withgrazing incidence mirrors such as in FIG. 8 is smaller in the verticaldimension than for pure tilted grating systems. A typical value is ofseveral mm by several cm. Nevertheless, one can stack multiple suchmirror/glancing incidence grating interferometers in order to obtain alarger field of view, similar to FIG. 6. This possibility has been infact demonstrated experimentally for conventional X-ray imaging in Ref.10, where tens of laterally graded multilayer mirrors have been stackedone upon the other to make a large area (˜10×20 cm) quasi-monochromaticradiographic system.

DETAILED DESCRIPTION REFERENCES

-   1. S.-A. Zhou and A. Brahme, Physica Medica 24 129 (2008)-   2. Momose A, Yashiro W, Takeda Y, Suzuki Y and Hattori T, Japanese    Journal of Applied Physics 45 5254 (2006)-   3. Pfeiffer F, Weitkamp T, Bunk O and David C, Nature Physics 2, 258    (2006)-   4. Tilman Donath, Franz Pfeiffer, Oliver Bunk, et al., Rev. Sci.    Instrum. 80, 053701 (2009)-   5. David C, Bruder J, Rohbeck T, Grunzweig C, Kottler C, Diaz A,    Bunk O and Pfeiffer F, Microelectronic Engineering 84, 1172 (2007)-   6. Reznikova E, Mohr J, Boerner M, Nazmov V, Jakobs P-J, Microsyst.    Technol. 14 1683 (2008)-   7. D. Stutman, M. Finkenthal, N. Moldovan, Applied Optics 49, 4677    (2010)-   8. D. Stutman, T. Beck, J. Carrino and C. Bingham, Phys. Med. Biol.    56, (5697) 2011-   9. Y. Park, S. Han, J. Chae, C. Kim, K. S. Chon, H.-K. Lee and D. S.    Han, Proc. SPIE 7258 Medical Imaging 2009: Physics of Medical    Imaging, 72583L (2009)-   10. M. Testorf, J. Jahns, N. A. Khilo, and A. M. Goncharenko, Opt.    Commun. 129, 167-172 (1996)-   11. Han Wen, Camille K Kemble, and Eric E. Bennett OPTICS EXPRESS    19, 25093 (2011)

FURTHER EMBODIMENTS AND EXAMPLES

The following examples analyze the angular sensitivity needed forrefraction enhanced imaging with the Talbot method and proposes ways tooptimize the Talbot setup for improved refraction based imaging withconventional X-ray sources. Even though we use examples from medical andhigh energy density (HED) plasma imaging, the conclusions apply also toother fields, such as material sciences, NDT, or security.

The Talbot interferometer is based on the Talbot effect, which consistsof the production of micro-fringe patterns by a ‘beam-splitter’ gratingilluminated by X-rays, at the so called Talbot distances d_(T)=m g₁²/8λ, where λ is the wavelength, g₁ is the grating period, and m=1, 3, 5. . . is the order of the pattern. The basic interferometer consists ofthe beam-splitter (typically a π-shift phase grating) followed by an‘analyzer’ absorption grating of period g₂ equal to that of the Talbotfringe pattern and placed at the magnified Talbot distanceD˜d_(T)/(1−d_(T)/L) from the beam-splitter, where L is the distancebetween the source and the beam-splitter (FIG. 10). When a refractiveobject is introduced in the X-ray beam the Talbot pattern is shifted,leading to intensity changes behind the analyzer approximatelyproportional to the angle of refraction of the X-rays. Since hard X-raysare deflected by only a few g-radians in low-Z matter, g₂ must be of theorder of a few μm and D of the order of the meter to achieve sufficientangular sensitivity. In addition, to make the interferometer work withextended, incoherent X-ray sources, a third, absorption grating havingperiod g₀=g2·L/D and openings of width s₀<g₀/2 is placed near thesource, effectively dividing it into an array of quasi-coherentmicro-sources. This choice of period and opening width ensures that theTalbot patterns from each micro-source constructively add at theanalyzer, for any L and D combination [13-15,19-21].

The interferometer is characterized by the angular width or resolutionW˜g₂/D, which determines its angular sensitivity S=1/W, and by the meanenergy <E>, and spectral width AE, of the region of high fringecontrast, which determine its spectral response. Typical angular widthsare in the 5-10 μ-radian range and typical contrast values are ≦few tensof percent when working with conventional X-ray sources [20,21]. Inaddition, as discussed in Ref. 19, the effective angular sensitivity ofthe Talbot interferometer S_(eff), decreases proportional to thedistance R between the beam-splitter and the object; for instance,S_(eff)=S·(1−R/D) if the object is placed behind the phase-grating as inFIG. 10. The decrease comes from the fact that the refraction angle‘seen’ by the beam-splitter at a distance R is smaller than that at theobject [19].

One can thus define an effective angular width for the Talbotinterferometer as W_(eff)=1/S_(eff) and summarize the two conditionsthat must be simultaneously met to achieve substantial refractioncontrast enhancement with the Talbot method: (i) high interferometercontrast and (ii) effective angular width comparable to the range ofrefraction angles produced by the object.

Mean energies possible with grating interferometers are up to a few tensof keV, with spectral widths ΔE/<E>˜1/m, where m is the Talbot order[13-15, 20-21]. The upper energy bound is due to technological limits inthe fabrication of thick, micron-period absorption gratings [22, 23].The optical transmission or throughput of the Talbot interferometer fordivergent and polychromatic light is much higher (up to 10-20%) than forcrystal ABI systems. The Talbot method can thus efficiently utilize thespectrally broad and divergent emission produced by conventional X-raysources. The field of view is limited by the practical grating size at<10×10 cm approximately.

While the Talbot method is attractive for practical applications, asabove mentioned the results so far indicate that its refraction contrastis lower than that of the crystal method. It is thus useful to brieflycompare the two methods in order to delineate the fundamentaldifferences. This can be done by comparing the ‘phase-scan’ intensitycurve in the Talbot method [14,15] with the rocking curve of theanalyzer crystal in the ABI method [5]; these curves play an equivalentrole in refraction based imaging as discussed in the following.

The phase-scan technique is illustrated with a numerical simulation inFIGS. 11A-11D. To compute refraction images we use throughout theseexamples the XWFP code in conjunction with the XOP database [24, 25].XWFP computes the X-ray wave propagation, including absorption,refraction and diffraction, through objects such as rods, spheres, andcavities, and through optical elements such as phase and absorptiongratings. The XOP database allows computing δ and β for materials ofarbitrary composition, by specifying the mass fraction for each elementand the mass density of the compound.

We simulated spectrally averaged refraction images for an interferometerhaving a ‘symmetric’ design in which L=D and gratings of equal period of10 μm. The absorption gratings had 60 μm thick gold bars and the phasegrating 25 μm thick Si bars, for a mean energy of 20 keV. Theinterferometer was set in the third Talbot order (L=D=1.2 m), with R=1cm (W_(eff)˜W=8.3 μ-radian)). We assumed the source is a 60 μm spot Wanode X-ray tube operated at 25 kV (<E>˜20 keV), exposure of 10 mA·s,and a detector having 20% quantum efficiency and 50 μm resolution. Astest object we used a 1 mm diameter Be rod in water medium, producingrefraction angles in the range <α_(m)=±4 μ-radian. A 100 μm diameterX-ray opaque Au wire was also included in the simulation to provide acontrast reference. The spectrally averaged images were obtained byweighting monochromatic images computed at 0.5 keV intervals with the Wtube power spectrum and by including statistical photon noise.

The phase-scan curve obtained by scanning the analyzer position in 30steps of size z=1 μm is shown in FIG. 11A. For comparison with thecrystal method we plotted the ordinate in units of angle spanned by thephase-scan, θ˜k·z/D, k=0, 1, . . . , with z the step size. The maxima ofthe phase-scan modulations represent the ‘bright-field’ (BF) intensityand the minima the ‘dark-field’ (DF) intensity [15]. The normalizeddifference between these intensities can be used to define theinterferometer contrast, V_(Talbot)=(I_(BF)−I_(DF))/(I_(BF)+I_(DF)).This definition is similar to that of the Talbot fringe contrast orvisibility [20,21], while characterizing the overall interferometercontrast. The computed contrast values in FIG. 11A match well thoseobtained experimentally with Talbot interferometers operated withconventional X-ray tubes [13-17].

FIG. 11B shows the raw, refraction enhanced image obtained at aninterferometer position in the middle of the quasi-linear portion of thephase-scan curve, as indicated by the arrow. Refraction contrast of ˜20%obtains at edges of the Be rod, showing that the Talbot method canproduce contrast enhancements of the order of α_(M)/W_(eff), evenwithout phase-scanning.

FIGS. 11C and 11D show the output of the phase retrieval procedure. FIG.11C shows the phase gradient or ‘pure refraction’ image, in which theintensity is proportional to the refraction angle, while FIG. 11D showsthe ‘pure attenuation’ image [14,15]. The analysis was done using theFourier method described in Ref. 15. FIGS. 11B to 11D illustrate thepotential of refraction based imaging: while the weakly absorbing Beobject is almost invisible in the attenuation image, it appears withgood contrast in the phase gradient and in the refraction enhancedimages.

To make a quantitative comparison between the Talbot method and thecrystal one we also plotted in FIG. 11A a Lorentzian of 1.5 μ-radianFWHM, approximating the typical rocking curve of the analyzer crystal inthe ABI method [5]. By comparing the angular width W˜g₂/D of the Talbotphase-scan modulation with the angular width of the crystal rockingcurve one can thus directly compare the angular sensitivity of the twomethods. An approximate comparison between the contrast of the twomethods can also be made by defining an equivalent ‘crystal contrast’V_(crystal) as above and by using as I_(BF) the intensity at the peak ofthe rocking curve and as I_(DF) the intensity in its wings, for instanceat one FWHM distance away from the peak.

Three basic differences between the two methods are apparent from thiscomparison:

-   -   First, the typical crystal angular width is several times        smaller than that of the Talbot interferometer (W˜8.5 μ-radian        in FIG. 11A).    -   Secondly, the equivalent crystal contrast is also substantially        higher, V_(crystal)˜67%, as compared to V_(Talbot)˜25%.    -   Thirdly, FIG. 11A shows that the Talbot interferometer works as        a periodic angular filter, while the crystal filters only a        narrow angular range. Thus, the Talbot interferometer does not        reject X-rays scattered at angles higher than its angular width,        while the crystal does. The rejection of scattered radiation is        deemed to be an important factor in the superior performance of        the ABI method [1-5].

This discussion raises two questions: (i) how does the typical angularwidth of the Talbot method compare to the range of refraction anglesexpected in applications, and (ii) how can the angular sensitivity andcontrast of the Talbot method be made closer to that of the crystalmethod. The first point is discussed in the following.

Range of X-Ray Refraction Angles in Practical Applications

To assess how the angular width of the Talbot method compares with theX-ray refraction angles encountered in typical applications weconsidered two practical examples: the refraction of hard X-rays in aHED plasma and the refraction in soft issues such as cartilage, tendonand muscle.

The Case of HED Plasma Radiography.

In the typical HED plasma radiography a micron sized X-ray backlighter(usually a laser produced plasma) illuminates a sub-mm, low-Z plasmatarget of many times the solid density, such as an imploding IFE(Inertial Fusion Energy) capsule. High spatial resolution requiresimaging at high magnification (M˜10-100) [11,26,27].

To estimate the refraction angles in IFE radiography we modeled theimploding capsule as concentric layers of Be and H having and 0.4 mm and0.3 mm diameter respectively, and 0.1 mm thickness and 6 g/cm³ densityeach. For the imaging setup we assumed a distance between thebacklighter and the capsule of 7.5 cm and L=D=2 m (R=1.9 m). In thissetup the beam-splitter could be sufficiently far from the implodingcapsule to survive the implosion when placed behind a protective filter[26,27]. However, since the imaged object is far from the beam-splitter,the effective angular sensitivity is reduced as above discussed, by thefactor (1−R/L)˜0.05.

FIG. 12 shows the range of refraction angles incident on thebeam-splitter for a typical backlighter energy of 22 keV (Ag K-α, [27]).As seen, while the refraction contrast enables one to discriminate theBe and H layers (otherwise invisible in the attenuation image), therange of refraction angles is small, α_(M)≦±1 μ-radian.

The Case of Soft Tissue Radiography.

Soft tissue imaging is one of the most investigated applications of theTalbot method. The synchrotron experiments show for instance that X-rayrefraction enables imaging of joint soft tissues such as cartilage ortendon, which are important in the diagnostic of arthritis [1,4,18]. Toestimate the typical refraction angles for soft tissues we assumed thecase of a small joint and used a simple numerical model or ‘phantom’ tocompute its attenuation and refraction angle profiles. The phantomconsisted of layers of materials simulating bone, cartilage, synovialfluid, connective tissue of the joint capsule, tendon, and skeletalmuscle (inset in FIG. 13), approximating the anatomy of a human proximalfinger joint. To compute δ and β for the joint soft tissues we used thecomposition and density of body tissues from the compilation by Woodardand White [28].

The refraction angles for the small joint phantom at 25 keV are shown inFIG. 13. As can be seen, with the exception of the bone/cartilage and ofthe tendon/muscle combinations, the range of refraction angles forcartilage, fluid and joint capsule is very small, α_(M) in the range ofa few tenths of a μ-radian. This is due to the small difference in indexof refraction between soft issues (e.g., several % for cartilage andjoint fluid). These very small refraction angles predicted by our modelare also in agreement with the synchrotron experiments; for instance,Shimao et al. estimated refraction angles in the range 0.1-0.4 μ-radianfor a human finger joint at 36 keV [18].

The conclusion from the above is that the substantially larger widthcharacteristic of Talbot interferometers, as well as their lowerintrinsic contrast, can make soft tissue imaging with conventional X-raysources challenging. A somewhat similar situation occurs in IFE DPCradiography for geometries where the beam-splitter is placed far fromthe target plasma. Ways must thus be explored to optimize the Talbotsetup for maximal angular sensitivity and contrast, as furtherdiscussed.

Optimization of the Talbot Setup for High Angular Sensitivity andContrast

With the notations in FIG. 10, in a magnifying geometry the angularwidth W of the Talbot interferometer is W˜g₂/D=M_(T) g₁/D∝λ/(m·g₁),where M_(T)=(L+D)/L is the Talbot magnification [19,20]. Thus, a firstway to decrease the angular width at a given wavelength is to increasethe Talbot period. However, this rapidly increases the interferometerlength, since the Talbot distance scales as the square of the period.Alternatively, one can increase the Talbot order m. However, since thewidth of the spectral region of high contrast scales as 1/m, thisapproach is also constrained by the use of a spectrally broad X-raysource, such as for instance a W anode tube.

The above relation shows that there are multiple combinations of gratingperiod, Talbot order and distances that can be used for a giveninterferometer length, Z=L+D. To find the values that maximize theangular sensitivity for a given system length we plotted the Talbotinterferometer equations as a function of the Talbot magnificationM_(T)=(L+D)/L, with the mean energy <E>, Talbot order m and the systemlength Z, as parameters. The results for <E>=25 keV, m=3, and Z=1.0,1.5, and 2 m are plotted in FIGS. 14A and 14B. R=5 cm was assumed in allcases. A first observation from FIG. 14A is that a small angular widthrequires a large interferometer length. A practical limit of a few m ishowever imposed for this length by mechanical stability considerationsand by the photon flux available from conventional X-ray sources.

Secondly, FIG. 14A shows that for a given system length the angularwidth is minimized in a ‘symmetrical’ Talbot setup, having L=D(M_(T)=2). The dependence of the periods g₀, g₁ and g₂ on M_(T) for Z=2m and m=3 are shown in FIG. 14B, indicating that the symmetrical setuphas also the practical advantage that all grating periods are equal andrelatively large. For instance g₀=g₁=g₂˜8 μm for Z=2 m, E=25 keV, m=3,which can be easily achieved in practice.

Thirdly, FIG. 14A shows that once the system length is fixed and thesymmetrical setup chosen, the only way to further increase the angularsensitivity is to increase the Talbot order. However, as mentioned, whenworking with spectrally broad X-ray sources there is a limit to how muchthe angular sensitivity can be increased in this way, due to thedecrease in spectrally averaged fringe contrast.

To illustrate this point, in FIG. 15 we plot the computed fringecontrast at increasing Talbot orders for a 2 m long symmetricinterferometer having <E>=25 keV. We assumed 55 μm thick gold source andanalyzer gratings and 33 μm thick Si phase grating. The source gratinghad openings of width s₀=g₀/2 (50% duty factor). The interferometercontrast is defined as above. The Talbot period was adjusted in eachorder to match the 2 m interferometer length. The contrast curves inFIG. 15 include also the geometrical broadening of the Talbot fringepattern by the finite source grating openings, simulated by convolvingthe Talbot pattern at the analyzer with a Gaussian of width s₀ [20,21].

For comparison we also plotted in FIG. 15 the spectrum of a W anodeX-ray tube at 35 kV, filtered with 1 mm Al and after traversing 20 mm ofsoft tissue. This approximates the spectrum incident on thebeam-splitter for a small biomedical object such as the above jointphantom. As can be seen, the overlap between the contrast curve and thebroad W anode spectrum rapidly decreases with increasing Talbot order.The spectrally averaged contrast is 32% for m=1, 27% for m=3, and 20%for m=5.

In conclusion, a practical configuration maximizing the angularsensitivity of the Talbot method is a symmetric setup having gratings ofequal period and length of around 2 m. In addition, the third Talbotorder offers a good compromise between angular sensitivity and contrastwhen using a spectrally broad source.

Nevertheless, as shown in FIG. 14A, the smallest angular widthachievable with a Talbot interferometer in a low order (m≦3) is stillseveral times larger than that of a crystal system. Thus, the only wayto achieve with the Talbot method angular sensitivity closer to that ofcrystal optics is to use higher Talbot orders. For instance, as shown inFIG. 14A, nearly 5 μ-radian angular width can be obtained with a 2 mlong interferometer in the 7th order.

At the same time, as shown in FIG. 15, as the Talbot order is increasedthe interferometer contrast curve ‘breaks’ into m narrow peaks that havedecreasing overlap with a broad source spectrum. Moreover, a detailedanalysis shows that the higher order contrast curves in FIG. 15 are in asense misleading, because the angular width changes with energy too.This is shown in FIG. 16 with plots of the computed Talbot pattern forthe central (25 keV) and the adjacent (19 keV and 37 keV, respectively)m=5 contrast peaks in FIG. 15. As can be seen, among the m=5 peaks onlythat at the design energy of 25 keV has both high contrast and smallangular width. The adjacent peaks are ‘harmonics’ that produce highcontrast Talbot patterns, but having twice the period of the pattern ofthe central peak. As such, although a broad source spectrum wouldoverlap with these side peaks, they would not contribute to theformation of the refraction image with the full angular sensitivity ofthe interferometer, but with half this value. In addition, depending onthe details of the imaged object, these side peaks could subtract fromthe effective refraction contrast produced by the central peak, insteadof adding to it.

In conclusion, our analysis shows that for interferometers of practicallength the angular width of the Talbot method is intrinsically limitedto values above 5 μ-radian approximately, which is higher than those ofcrystal systems (<1.5 μ-radian). In addition, to achieve its smallestpossible angular width the Talbot interferometer must be operated in ahigh order, in which case it is not optimal to use a broad sourcespectrum, since the effective contrast substantially decreases.

The solution to simultaneously maximize the angular sensitivity and theeffective contrast of Talbot method is thus to work in a high order(m>5), while using a quasi-monochromatic X-ray spectrum of widthAE/<E>≦1/m˜15-20%. Possible ways to do this are described in thefollowing.

Talbot Interferometry with Quasi-Monochromatic Spectra

K-Line Spectra Filtered with K-Edge Absorbers.

The simplest method to obtain a quasi-monochromatic spectrum is to use abright K-line emitter, such as a Mo or Rh anode tube for biomedicalapplications or an Ag K-α backlighter for HED plasma radiography, and tofilter the emission with a K-edge absorber of the same atomic number asthe emitter.

The spectrum of a Rh anode tube at 40 kVp filtered with 30 μm Rhabsorber and after transmission through 20 mm of soft tissue is shown inFIG. 17A. Also shown in FIG. 17A is the computed contrast of a symmetric2 m Talbot interferometer having 6 μm period, 55 μm thick Au source andanalyzer gratings, s₀=g₀/2, Si phase grating optimized for 20 keV meanenergy, and operated in the 7th order. As can be seen, the K-edgefiltered spectrum is dominated by the strong Rh K-α line at 20 keV,which matches closely the peak of the contrast curve in the 7th order. Asimilar good match can be produced for the Mo K-α line at 17.5 keV.

The increase in refraction contrast possible using high Talbot ordersand K-line/K-edge filtered spectra is illustrated with computedrefraction enhanced images of the joint phantom in FIGS. 18A-18D. Weassumed the above 2 m interferometer, a 50 μm pixel detector, and anexposure of 50 mA·s with a Rh anode tube at 40 kVp, producing a meandetector count of ˜100 per pixel. The refraction enhanced images arecomputed for an interferometer phasing at mid-distance between thebright and dark field settings, which as illustrated in FIG. 11Bmaximizes the refraction contrast.

FIG. 18A shows as a reference the image obtained assuming the W anodetube spectrum in FIG. 15 and operation in the third Talbot order,optimal for this spectrum. As can be seen, due to insufficient angularsensitivity, the refraction contrast enhancement is too faint to beuseful in practice without resorting to phase-scanning and/or CT, whichwould require multiple exposures.

FIG. 18B shows that the single exposure contrast can be substantiallyincreased however by using the interferometer in the 7th order and theK-edge filtered Rh spectrum; the cartilage, joint fluid and connectivecapsule are clearly delineated in this case. The relative intensityvariation or contrast at the cartilage fluid interface for instance isaround 20%.

A HED plasma example of quasi-monochromatic imaging in a high Talbotorder is illustrated in FIG. 19, which shows a Moiré fringe image ordeflectogram of the IFE capsule modeled in FIG. 12. The use of Moirédeflectometry for density profile diagnostic in HED plasmas wasdemonstrated at the NOVA facility using backlighting with an XUV laserand focusing optics [29]. We assumed a symmetric interferometer of 4 mlength and 10 μm period operated in the 5th Talbot order, a detectorwith 50 μm pixels, and illumination with a Ag K-α backlighter spectrumfiltered with 50 μm Ag. The clear Moiré fringe shifts at the location ofthe Be ablator and H fuel layer in FIG. 19 indicate that using theTalbot method with quasi-monochromatic backlighting would provide asimple density profile diagnostic for the capsule, without the need forX-ray lasers or focusing optics.

Mirror Filtered Slot-Scan Talbot Interferometers.

While offering the simplest approach, the contrast increase possiblewith K-edge filtering is limited, since as shown in FIG. 17A asubstantial fraction of photons is emitted at energies above the K-αenergy, where the interferometer has low angular sensitivity. Inaddition, the choice of bright K-line sources in the range of a few tensof keV is limited (e.g., only Mo or Rh anode tubes for medicalapplications).

To further increase the sensitivity and contrast of the Talbot methodand to broaden the range of possible interferometer energies we proposeto use X-ray mirrors or reflectors to shape the source spectrum. Theprinciple of the method is sketched in FIG. 20. A grazing incidencemirror is placed near the source grating and a slot collimator selectsonly the reflected beam.

There are several choices for the filtering mirror. A first possibilityis to use total reflection mirrors. These are simply made of a thinhigh-Z film (e.g., Au, Ta, Pt) deposited on a low-Z substrate and canreflect with high efficiency (>60-80%) hard X-rays incident below thecritical reflection angle [30]. The sharp energy cutoff due to the totalreflection effect can be used to efficiently filter out high energyphotons. This is illustrated in FIG. 17B with the computed Rh tubespectrum at 40 kVp, filtered with a 30 μm Rh absorber followed byreflection on a Pt mirror at 3.5 mrad incidence angle. The mirror wasassumed to have 3 Å surface roughness. As can be seen, the parasiticradiation above about 22 keV is completely suppressed, while theradiation in the useful Rh K-α band is efficiently transmitted.

The image of the joint phantom obtained assuming this spectrum ispresented in FIG. 18C, showing that suppressing the parasitic band ofhigh energy photons strongly increases the refraction contrast, with theintensity contrast at the cartilage fluid interface reaching ˜35%.Another practical benefit of the mirror filtering technique is that itwould allow increasing the brightness of the K-α band by increasing thetube voltage, since the photons above the K-α band are not reflected. Itis advantageous to increase the K-α brightness by increasing the voltagerather than the current, since it scales as the voltage to the power of1.5-1.6.

Another possibility with the mirror technique is to use laterally gradedmultilayer mirrors as narrow band, high throughput spectral filters.These are synthetic Bragg reflectors for which the period varies alongthe length, enabling it to reflect a narrow range of wavelengths overthe entire length of a planar mirror [31]. Recent experimentsdemonstrate that at incidence angles of several milli-radians suchmirrors can efficiently reflect X-rays up to tens of KeV. For instance,Park et al. demonstrated efficient production (>50% reflectivity) ofquasi-monochromatic X-ray bands using a conventional rotating anodeX-ray tube and a 100 mm long graded multilayer with period varyingbetween 32 and 38 Å [32]. The mean X-ray energy/bandwidth could bevaried between 20 keV/15% and 40 KeV/7.5%. Curved HOPG (highly orderedpyrolytic graphite) reflectors could also be used to produce nearlymonochromatic radiation from conventional X-ray sources, as demonstratedwith a Mo K-α mammographic system by Lawaczeck et al. [33].

Using such reflectors, narrow K-α spectra can be produced that wouldfurther increase the refraction contrast of the Talbot method. This isillustrated in FIG. 18D assuming illumination of the joint phantom withphotons in a 4 keV wide band centered on the Rh K-α energy. The contrastat the cartilage fluid interface reaches nearly 50% in this case. (Notethat due to the narrower spectrum the K-α intensity in FIG. 18D wasassumed to increase by a factor of ˜3 to achieve the same photon countas in FIGS. 18B and 18C; as above discussed, this could be simply doneby increasing the tube voltage from 40 to about 60 kV.)

The constraint in the mirror filtering method is that the field of view(FOV) height perpendicular to the mirror plane (vertical in FIG. 20) islimited to values H˜Δαd at the object location, with Δα the differencebetween the maximum and the minimum incidence angle on the mirror and dthe distance between the mirror and the object. For total reflectionmirrors Δα is constrained in turn by the acceptable variation in highenergy cutoff across the length of the mirror. For instance, assuming aRh anode spectrum at 60 kVp and a Pt mirror at 3.5 milli-radian centralincidence angle, Δα of ˜1 milli-radian would correspond to a cutoffenergy variation between 22 keV and 28 keV, which would still allowobtaining high refraction contrast as in FIG. 18C. The vertical FOV atthe object will thus be limited to H˜1 mm for a 2 m long interferometerhaving d˜L, as in FIG. 20. In the perpendicular direction the FOV islimited only by the available grating width, since large area X-raymirrors can nowadays be easily produced.

With laterally graded multilayers the field of view height could besubstantially larger, however, since the only limiting factor is theBragg angle variation along the mirror. For instance, assuming themirror parameters in Ref. 32, H would increase to ˜2.5 mm for a 2 m longinterferometer. Further on, using curved optics the field of view couldbe even larger; for instance, using a 50 mm long crystal with 480 mmcurvature radius placed at 50 mm from the source Lawaczeck et al.achieved a 10 mm high FOV for Mo K-αradiation, at 550 mm distance fromthe source [33]. For a 2 m long symmetric Talbot interferometer thiswould translate into a FOV height of ˜15 mm.

Nonetheless, to image large objects, the mirror filtered Talbotinterferometer would need to work in a slot-scan mode, in which eitherthe object or the interferometer field of view is scanned vertically inFIG. 20. This would require, in principle, longer measurement times thanpossible with a large field of view, ‘cone-beam’ system. We note howeverthat a compensating advantage of the slot-scan geometry could be thestrong reduction in large angle scattered radiation reaching thedetector. As demonstrated by slot-scan medical systems this reductionsubstantially improves the overall image contrast [32-34]. In addition,using a quasi-monochromatic spectrum has the advantage of decreasing theradiation dose, since only the wavelength useful for imaging is incidenton the object [33,34]. The slot-scan Talbot systems would also closerresemble the crystal ABI systems, which as above discussed also rejectthe large angle scattered radiation. Lastly, the measurement time of amirror filtered slot-scan system could be drastically shortened by usingmultiple, stacked reflectors. This was demonstrated by Park et al., whoused an array of stacked multilayer mirrors to achieve scan times ofless than 1 s for an image of ˜200 mm×240 mm size [32].

The mirror filtering could enable also extending the range of energybands available for quasi-monochromatic Talbot interferometry. Thiscould be done using narrow band-pass mirrors in combination with abright continuum source, such as a rotating W anode tube. A first way toobtain narrow energy bands could be to use depth graded multilayermirrors. These are multilayers for which the period varies with thedepth, enabling to efficiently produce energy bands of widthΔE/<E>˜10-15%, for X-rays up to several tens of keV energy [35,36].

In addition, a simple and tunable band-pass filter could be made usingtwo total reflection mirrors. This dual-mirror filter design is sketchedin FIG. 3A and expands on a filtering technique demonstrated at thesynchrotrons (the ‘transmission mirror’) [37,38]. The first mirror has ahigh-Z metallic film deposited on a thin (few μm) low-Z membrane. Totalreflection on this mirror rejects the low energy part of the spectrum,while the high energy part is transmitted through the thin membrane withlittle attenuation. The radiation transmitted by the first mirror isthen low-pass filtered by a second total reflection mirror. FIG. 3Bshows an example of the spectral response possible with this design,indicating that band-pass of the order of 15-20% could be achieved forenergies of up to several tens of keV. These energy bands would in turnmatch well the contrast of Talbot interferometers in high orders, asalso illustrated in FIG. 3B.

Lastly, a further improvement to the mirror filtered interferometerdesign would be to combine the source grating and the filter mirror in asingle optical element, using the micro-periodic mirror concept wedescribed in Ref. 30. These are total reflection ‘mirror gratings’ madeby patterning a low-Z substrate with thin (−500 Å), periodic strips ofhigh-Z metal. As shown in Ref. 30, the difference in reflectivitybetween the high-Z strips and the low-Z substrate enables one to producehigh contrast (up to ˜80%) reflection gratings for X-ray energies up toseveral tens of keV. Thus, in addition to simplifying the optical setup,the use of a micro-periodic mirror instead of the ‘source’ grating wouldallow increasing the interferometer contrast at high energy, since themirror would be the equivalent a very thick absorption grating.

This possibility is illustrated in FIGS. 21A-21B with calculations ofrefraction enhanced images for a large joint phantom. The phantom hasthe same layout as the one in FIG. 13, but with dimensions typical of aknee joint (15 cm muscle diameter, 1.5 mm thick cartilage, fluid andconnective tissue layers, 35 mm bone diameter and 6 mm diameter tendon).As the source, we assumed a W anode tube of 0.3 mm spot operated at 70kVp (typical of knee radiography) and filtered with 0.12 mm Cu and 2 mmAl. The detector had 100 μm pixels.

FIG. 21A shows the image obtained assuming a 2.2 m long symmetricinterferometer of 45 keV mean energy and 5 μm period, operated in the5th order, and using 100 μm thick source and analyzer gratings, with asource grating duty factor of 33%. The photons above ˜50 keV are cut bya Pt mirror at 1.8 milli-radian incidence angle. As can be seen, therefraction contrast for soft tissues is poor because the absorptioncontrast between the bars and the openings of the source gratingdecreases rapidly for X-rays above a few tens of keV.

FIG. 21B shows the image obtained assuming instead of the source gratinga micro-periodic Pt mirror, having 33% duty factor and 80% reflectioncontrast between the reflecting and non-reflecting strips, independentof energy [30]. As can be seen, replacing the grating with amicro-periodic mirror would strongly improve the refraction contrast athigh energy, making visible all soft tissues in the large joint. Lastly,to achieve the maximum possible refraction contrast the source gratingcould be replaced with a micro-periodically patterned multilayer mirroror possibly a patterned HOPG crystal, for near monochromaticdifferential phase-contrast imaging at high energy.

CONCLUSIONS

Our analysis shows that while Talbot interferometry is a simpletechnique for refraction based imaging, its angular sensitivity andcontrast should be carefully optimized in order to compete with those ofthe crystal method. This is particularly critical for demandingapplications such as soft tissue imaging or high energy density plasmadiagnostic, where the refraction angles can be in the sub μ-radianrange. A practical way to simultaneously maximize the angularsensitivity and contrast of the Talbot method is to use a symmetricinterferometer setup with a quasi-monochromatic source spectrum. Severalsolutions are described for shaping the source spectrum, ranging fromK-edge absorption filters to reflection on grazing incidence mirrors.The calculations suggest that using such filtering strong refractioncontrast could be obtained for low-Z objects at energies up to a fewtens of keV. The combination of Talbot gratings with band-pass mirrorsand/or micro-periodic mirrors appears also attractive for extending theTalbot method to higher X-ray energy.

REFERENCES

-   1. S.-A. Zhou and A. Brahme, Physica Medica 24 129 (2008)-   2. Keyriläinen J, Bravin A, Fernandez M, Tenhunen M, Virkkunen P and    Suortti P, Acta Radiologica 8 866 (2010)-   3. D Chapman, W Thomlinson, R E Johnston, D Washburn, E Pisano, N    Gmür, Z Zhong, R Menk, F Arfelli and D Sayers Phys. Med. Biol. 42    2015 (1997)-   4. Carol Muehleman, Jun Li, Zhong Zhong, Jovan G Brankov, and Miles    N Wernick, J Anat. 2006 208, 115-124-   5. Suhonen H., Fernandez M., Bravin A., Keyrilainen J. and Suorttia    P., J. Synchrotron Rad. 14, 512 (2007)-   6. Arfelli F., Rigon L. and Menk R. H., Phys. Med. Biol. 55 1643    (2010)-   7. R. A. Lewis, Phys. Med. Biol. 49 3573 (2004)-   8. A. W. Stevenson, T. E. Gureyev, D. Paganin, S. W. Wilkins, T.    Weitkamp, A. Snigirev, C. Rau, I. Snigireva, H. S. Youn, I. P.    Dolbnya, W. Yun, B. Lai, R. F. Garrett, D. J. Cookson, K. Hyodo, M.    Ando, Nuclear Instruments and Methods in Physics Research B 199 427    (2003)-   9. S. Mayo, R. Evans, F. Chen and R. Lagerstrom, Journal of Physics:    Conference Series 186, 012105 (2009)-   10. Brey E M, Appel A, Chiu Y C, Zhong Z, Cheng M H, Engel H,    Anastasio M A, Tissue Eng. Part C Methods. 16, 1597 (2010)-   11. Jeffrey A. Koch, Otto L. Landen, Bernard J. Kozioziemski,    Nobuhiko Izumi, Eduard L. Dewald, Jay D. Salmonson, and Bruce A.    Hammel, J. Appl. Phys. 105, 113112 (2009)-   12. D. Stutman, M. Finkenthal and N. Moldovan, Rev. Sci. Instrum.    81, 10E504 (2010)-   13. Momose A, Yashiro W, Takeda Y, Suzuki Y and Hattori T, Japanese    Journal of Applied Physics 45 5254 (2006)-   14. Pfeiffer F, Weitkamp T, Bunk O and David C, Nature Physics 2,    258 (2006)-   15. Pfeiffer F, Bech M, Bunk O, Kraft P, Eikenberry E F, Bronnimann    Ch, Grunzweig C and David C, Nature Materials 7, 134 (2008)-   16. Bech M, H Jensen T H, Feidenhans R, Bunk O, David C and Pfeiffer    F, Phys. Med. Biol. 54 2747 (2009)-   17. Donath T, Pfeiffer F, Bunk O, Grünzweig C, Eckhard H, Popescu S,    Peter V and David C, Investigative Radiology 45, 445 (2010)-   18. Shimao D, Kunisada T, Sugiyama H, Ando M, European Journal of    Radiology 68 S27 (2008)-   19. Donath T, Chabior M, Pfeiffer F, J. Appl. Phys. 106 054703    (2009)-   20. Weitkamp T, David C, Kottler C, Bunk O and Pfeiffer F, Proc.    SPIE vol 6318, Developments in X-Ray Tomography V, 28 (2006)-   21. Engelhardt M, Kottler C, Bunk O, David C, Schroer C, Baumann J,    Schuster M, Pfeiffer F., Journal of Microscopy 232, 145 (2008)-   22. David C, Bruder J, Rohbeck T, Grunzweig C, Kottler C, Diaz A,    Bunk O and Pfeiffer F, Microelectronic Engineering 84, 1172 (2007)-   23. Reznikova E, Mohr J, Boerner M, Nazmov V, Jakobs P-J, Microsyst.    Technol. 14 1683 (2008)-   24. Weitkamp T, Proc. SPIE vol 5536 Advances in Computational    Methods for X-Ray and Neutron Optics, 181 (2004)-   25. Sanchez del Rio M and Dejus R J, Proc. SPIE vol 3448 Crystal and    Multilayer Optics, 340 (1998)-   26. H.-S. Park, B. R. Maddox, E. Giraldez, et al., Physics of    Plasmas 15, 07270 (2008)-   27. R. Tommasini, LLNL Report, LLNL-TR-429373, 2010-   28. Woodard H Q and White D R, The British Journal of Radiology 59,    1209 (1986)-   29. D. Ress, L. B. DaSilva, R. A. London, J. E. Trebes, and R. A.    Lerche, Rev. Sci. Instrum. 66, 579 (1995)-   30. D. Stutman, M. Finkenthal, N. Moldovan, Applied Optics 49, 4677    (2010)-   31. M. Schuster, H. Gael, L. Brugemann, D. Bahr, F. Burgazy, C.    Michaelsen, C. M. Stormer, C P. Ricardo, C R. Dietsch, T. Holz    and H. Mai, Proc. SPIE vol 3767 EUV, X-Ray, and Neutron Optics and    Sources, 183 (1999)-   32. Y. Park, S. Han, J. Chae, C. Kim, K. S. Chon, H.-K. Lee    and D. S. Han, Proc. SPIE 7258 Medical Imaging 2009: Physics of    Medical Imaging, 72583L (2009)-   33. R. Lawaczeck, V. Arkadiev, F. Diekmann, and M. Krumrey,    Investigative Radiology 40, 33 (2005)-   34. K Hussein, C L Vaughan and T S Douglas, Phys. Med. Biol. 54 1533    (2009)-   35. K. D. Joensen, P. Hoghoj, F. Christensen, P. Gorenstein, J.    Susini, E. Ziegler, A. Freund, J. Wood, Proc. SPIE vol 2011    Multilayer and Grazing Incidence X-Ray/EUV Optics II, 360 (1994)-   36. A. Rack, T. Weitkamp, M. Riotte, T. Rack, R. Dietsch, T.    Holz, M. Kramer, F. Siewert, M. Meduna, Ch. Morawe, P. Cloetens, E.    Ziegler, Proc. SPIE Vol. 7802, Advances in X-Ray/EUV Optics and    Components V, 78020M-1 (2010)-   37. S. Cornaby and D. H. Bilderback, J. Synchrotron Rad. 15, 371    (2008)-   38. A. Iida, T. Matsushita, and Y. Gohshi, Nucl. Instrum. Meth.    Phys. Res. A235, 597 (1985)

The embodiments illustrated and discussed in this specification areintended only to teach those skilled in the art how to make and use theinvention. In describing embodiments of the invention, specificterminology is employed for the sake of clarity. However, the inventionis not intended to be limited to the specific terminology so selected.The above-described embodiments of the invention may be modified orvaried, without departing from the invention, as appreciated by thoseskilled in the art in light of the above teachings. It is therefore tobe understood that, within the scope of the claims and theirequivalents, the invention may be practiced otherwise than asspecifically described.

1. A differential phase contrast X-ray imaging system, comprising: anX-ray illumination system; a beam splitter arranged in an optical pathof said X-ray illumination system; and a detection system arranged in anoptical path to detect X-rays after passing through said beam splitter,said detection system comprising an X-ray detection component, whereinsaid beam splitter comprises a splitter grating arranged to intercept anincident X-ray beam and provide an interference pattern of X-rays,wherein said detection system comprises an analyzer grating arranged tointercept and block at least portions of said interference pattern ofX-rays prior to reaching said X-ray detection component, wherein saidanalyzer grating has a longitudinal dimension, a lateral dimension thatis orthogonal to said longitudinal dimension and a transverse dimensionthat is orthogonal to said longitudinal and lateral dimensions, saidanalyzer grating comprising a pattern of optically dense regions eachhaving a longest dimension along said longitudinal dimension and beingspaced substantially parallel to each other in said lateral dimensionsuch that there are optically rare regions between adjacent opticallydense regions, wherein each optically dense region has a depth in saidtransverse dimension that is smaller than a length in said longitudinaldimension, wherein said analyzer grating is arranged with saidlongitudinal dimension at a shallow angle relative to incident X-rays,and wherein said shallow angle is less than 30 degrees.
 2. Adifferential phase contrast X-ray imaging system according to claim 1,wherein each optically dense region has a depth in said transversedimension that is smaller than a length in said longitudinal dimensionby at least a factor of two.
 3. A differential phase contrast X-rayimaging system according to claim 1, wherein each optically dense regionhas a depth in said transverse dimension that is smaller than a lengthin said longitudinal dimension by at least a factor of ten.
 4. Adifferential phase contrast X-ray imaging system according to claim 1,wherein each optically dense region has a depth in said transversedimension that is smaller than a length in said longitudinal dimensionby at least a factor of one hundred.
 5. A differential phase contrastX-ray imaging system according to claim 1, wherein said shallow angle isless than 25 degrees and greater than 3 degrees.
 6. A differential phasecontrast X-ray imaging system according to claim 1, wherein said shallowangle is less than 15 degrees and greater than 5 degrees.
 7. Adifferential phase contrast X-ray imaging system according to claim 1,wherein said splitter grating is a reflection grating.
 8. A differentialphase contrast X-ray imaging system according to claim 1, wherein saidsplitter grating is a transmission grating.
 9. A differential phasecontrast X-ray imaging system according to claim 8, wherein saidsplitter grating has a longitudinal dimension, a lateral dimension thatis orthogonal to said longitudinal dimension and a transverse dimensionthat is orthogonal to said longitudinal and lateral dimensions, saidsplitter grating comprising a pattern of optically dense regions eachhaving a longest dimension along said longitudinal dimension and beingspaced substantially parallel to each other in said lateral dimensionsuch that there are optically rare regions between adjacent opticallydense regions, wherein each optically dense region has a depth in saidtransverse dimension that is smaller than a length in said longitudinaldimension, wherein said splitter grating is arranged with saidlongitudinal dimension at a shallow angle relative to incident X-rays,and wherein said shallow angle is less than 30 degrees.
 10. Adifferential phase contrast X-ray imaging system according to claim 1,wherein said X-ray illumination system comprises: an X-ray source, and asource grating arranged in an optical path between said X-ray source andsaid beam splitter, wherein said source grating provides a plurality ofsubstantially coherent X-ray beams.
 11. A differential phase contrastX-ray imaging system according to claim 1, wherein said X-rayillumination system comprises: a poly-energetic X-ray source, and aband-pass filter arranged in an optical path of X-rays from saidpoly-energetic X-ray source, wherein said band-pass filter allows X-rayswithin a band of energies to pass more strongly than X-rays outside saidband of energies.
 12. A differential phase contrast X-ray imaging systemaccording to claim 11, wherein said band-pass filter comprises: ahigh-pass X-ray mirror that reflects a first portion of an incident beamof X-rays that have energies less than a lower pass-band energy andallows a second portion of said incident beam of X-rays to passtherethrough, a first beam stop arranged to intercept and at leastattenuate said first portion of said incident beam of X-rays that haveenergies less than said lower pass-band energy, a low-pass X-ray mirrorthat reflects a portion of said second portion of said incident beam ofX-rays after passing through said high-pass X-ray mirror that haveenergies less than a upper pass-band energy, and a second beam stoparranged to intercept and at least attenuate X-rays that miss saidhigh-pass X-ray mirror prior to reaching said second beam stop, whereinsaid first and second beam stops are arranged to allow a beam of X-rayshaving energies between said upper pass-band energy and said lowerpass-band energy to pass therethrough.
 13. A differential phase contrastX-ray imaging system according to claim 12, wherein said low-pass X-raymirror is a membrane X-ray mirror comprising a reflecting layer thatcomprises a high-Z material on a support layer that comprises a low-Zmaterial, wherein Z is an atomic number, wherein said high-Z materialincludes atomic elements with Z at least 42, and wherein said low-Zmaterial includes atomic elements with Z less than
 14. 14. Adifferential phase contrast X-ray imaging system according to claim 1,wherein said splitter grating and said analyzer grating are arrangedwith a separation determined according to Talbot-Lau conditions.
 15. Adifferential phase contrast X-ray imaging system according to claim 1,wherein said splitter grating and said analyzer grating have gratingpatterns determined according to Talbot-Lau conditions.
 16. An X-rayillumination system, comprising: a poly-energetic X-ray source; and aband-pass filter arranged in an optical path of X-rays from saidpoly-energetic X-ray source, wherein said band-pass filter allows X-rayswithin a band of energies to pass more strongly than X-rays outside saidband of energies.
 17. An X-ray illumination system according to claim16, wherein said band-pass filter comprises: a high-pass X-ray mirrorthat reflects a first portion of an incident beam of X-rays that haveenergies less than a lower pass-band energy and allows a second portionof said incident beam of X-rays to pass therethrough, a first beam stoparranged to intercept and at least attenuate said first portion of saidincident beam of X-rays that have energies less than said lowerpass-band energy, a low-pass X-ray mirror that reflects a portion ofsaid second portion of said incident beam of X-rays after passingthrough said high-pass X-ray mirror that have energies less than a upperpass-band energy, and a second beam stop arranged to intercept and atleast attenuate X-rays that miss said high-pass X-ray mirror prior toreaching said second beam stop, wherein said first and second beam stopsare arranged to allow a beam of X-rays having energies between saidupper pass-band energy and said lower pass-band energy to passtherethrough.
 18. An X-ray illumination system according to claim 17,wherein said low-pass X-ray mirror is a membrane X-ray mirror comprisinga reflecting layer that comprises a high-Z material on a support layerthat comprises a low-Z material, wherein Z is an atomic number, whereinsaid high-Z material includes atomic elements with Z at least 42, andwherein said low-Z material includes atomic elements with Z less than14.